1. Field of the Invention
The present invention relates to an optical signal amplification apparatus and an optical fiber transmission system employing the optical signal amplification apparatus. More specifically, the present invention relates to an optical signal amplification apparatus including a plurality of amplifiers connected in series and an optical fiber transmission system employing such an optical signal amplification apparatus.
2. Description of the Related Art
In recent years, an erbium-doped optical fiber amplifier capable of directly amplifying light of a 1550 nm band has been realized. With this realization, an analog optical fiber transmission system employing such an optical fiber amplifier has been studied and developed.
FIG. 20 shows a configuration of an analog optical fiber transmission system of a 1550 nm band employing a conventional optical fiber amplifier. The system includes a DFB (distributed feedback) semiconductor laser 110 having an oscillation wavelength of 1550 nm, an erbium-doped optical fiber amplifier 120 pumped by a semiconductor laser having an oscillation wavelength of 1480 nm, a 1.times.10 optical coupler 130, a dispersion shifted optical fiber 140 of a 1550 nm band, and a light-receiving element 150 made of an InGaAs photodiode or an avalanche photodiode.
In the DFB semiconductor laser 110, the power of signal light is modulated according to an AM-FDM (frequency division multiplexer) or FM-FDM analog signal. Signal light output from the DFB semiconductor laser 110 is amplified by the optical fiber amplifier 120, and branched by the optical coupler 130. Thereafter, the signal light is transmitted through the dispersion shifted optical fiber 140 and then input into the light-receiving element 150.
Since the erbium-doped optical fiber amplifier 120 has a high gain in the 1550 nm band, it is possible to construct an entirely optical distribution system where a signal is optically transmitted through the entire route from a head end to the respective users' homes. This makes it possible to distribute images covering 100 channels or more and high-definition TV images.
In the above optical fiber transmission system employing the fiber optical amplifier, it is known that, when AM-FDM or FM-FDM analog signal light is transmitted therein, a distortion of the signal light, especially, a distortion component based on second-order nonlinearity called a composite second order (CSO), is increased during the transmission through the optical fiber amplifier. This is reported, for example, by Kikushima et al. "IEEE Photonics Technology Letters" 1991, pp. 945-947.
When the power of signal light output from a semiconductor laser is intended to be analog-modulated, not only the power of the signal light but also the frequency thereof are modulated. This is called chirping. When the signal light having a modulated frequency is amplified by the optical fiber amplifier having a gain dependent on the wavelength of the signal light, the variation in the frequency of the signal light brings about a variation in the power thereof. This results in shifting of the power of the signal light from a desired level. In other words, since the gain of the optical fiber amplifier has a great wavelength-dependency (a gain tilt) in the wavelength band of signal light (for example, the 1550 nm band), the variation in the frequency of the signal light causes a shift of the power thereof from a desired level.
Such a distortion generated by one optical fiber amplifier is possibly multiplied when the signal light is transmitted through a plurality of optical fiber amplifiers connected in series.